1. Introduction
The Jeddah coast is a part of the Red Sea, which is a semi-enclosed warm water body separating Asia from Africa and surrounded by nine countries [
1]. Historically, the Red Sea has been used as the main route for merchant caravans from Europe to India and Eastern Asia and is still a major transportation routes, joining several of the world’s major ocean shipping routes. Despite such heavy transit, it is relatively unpolluted, apart from localized areas [
2]. With increasing coastal development, associated pollution is a rising concern for the region. Sediments can directly interact with other environmental media across numerous spatial and temporal scales [
3]. Coastal sediments of Jeddah are, therefore, attractive for scholars to study heavy metal enrichment due to the sensitivity of these sediments to industrial and anthropogenic activities [
4,
5].
Heavy metal sourcing to the local marine environment could be anthropogenic or natural, including pollution by oil spills, wastewater discharge, effluents of desalination plants, construction activities, and marine traffic [
5,
6,
7,
8]. According to Skaldina et al., [
9] industry, transport, refuse burning, and power generation are considered as significant sources of heavy metal introduction into different environmental media. Direct or indirect wastewater discharge with its heavy metal load, leads to an increase in environmental contaminants in the associated marine environment [
10,
11]. Prior work showed that anthropogenic activities clearly affect nearby sediments according to the rapid industrial transformation along the coastal area of Saudi Arabia [
12,
13].
The concentrations of heavy metals in sediments are vital indicators to identify and classify the degree of pollution for any area of concern [
14]. The concentrations of heavy metals in seawater are governed by suspended particulate matter and, ultimately, sediments [
4,
12]. Typically, suspended metals bind to particulate matter once discharged into aquatic systems; they settle and are sorbed by sediments [
15]. While this is a complex series of processes, as shown through heavy metal sediment concentrations’ correlation with pH, conductivity, salinity, and the availability of organic matter [
16], sediment concentrations are linked to aquatic concentrations. Both aqueous and sediment metals can be bioavailable. Some heavy metals such as lead, cadmium, mercury, chromium, and arsenic are identified to be toxic for living organisms, whereas copper, manganese, sodium, iron, and zinc are essential metals, but can be toxic if they exceed permissible levels [
17,
18].
A number of studies have investigated the surface sediments of the Red Sea and showed variable heavy metal contamination. Badr et al. [
8], studied heavy metals in Red Sea sediments near Jeddah and reported that Jeddah has the most contaminated sediments in comparison with the other industrial areas along the Red Sea coast. The highest concentrations of Cu, Zn, Cd, Ni, Cr, and Pb were recorded in the Al-Arbaeen and Al-Shabab inlets (central Jeddah coast), indicating their possible anthropogenic sourcing [
19]. Pan et al. [
5] reported that Zn, Cu, and Pb were high in sediment samples collected from near the Jeddah fish market. Moreover, Al-Mur et al. [
6] reported the same conclusions for sediments near Jeddah, where the highest enrichment was found for metals such as Mn, Cr, Pb, Zn, and Cu. Bantan et al. [
20] recorded high concentrations of the same heavy metals on the coastline of the Southern Corniche of Jeddah and attributed those concentrations to the influx of domestic and industrial wastewater into this zone.
The study area, including locations along the North, Middle, and South Jeddah coast (
Figure 1), is highly urbanized and rich with potential anthropogenic sources of contamination, especially the industrial activities of the Industrial City located along the southern part of Jeddah coast. The Jeddah coast is connected to industrial and sewage treatment plants that may act as sources of contaminants to the region [
21,
22]. Therefore, a comprehensive evaluation of heavy metals contamination in sediments of the entire Jeddah shoreline needs to be covered. The specific aims of this work are to: (i) investigate of the state of heavy metal pollution along Jeddah’s coast by measuring the total metal concentrations in the sediments; (ii) quantify the extent of pollution in sediments using the Geo-accumulation indices (Igeo), Enrichment Factor (EF), and Contamination Factor (CF); and (iii) determine the associated risk by using Pollution Load Index (PLI), potential ecological risk (PERI), and Potential Toxicity Response Index (RI).
Coastal areas are considered sequestration points for various pollutants generated from commercial and urban activities. Different human-induced pollutants are typically delivered to coastal sediments through atmospheric transport and fluvial processes. A higher concentration of contaminants, including heavy metals, leads to deteriorating environmental parameters, including those gauging marine ecosystem health. The state of heavy metals and their distribution in coastal sediments, therefore, needs to be identified by applying comprehensive indices for categorizing the pollution level to inform appropriate future management decisions. The data and interpretation provided here will yield valuable benchmarks, not just for the immediate area, but for the region as a whole and to similar tropical marine ecosystems worldwide.
4. Discussion
The Northern location of the Jeddah coast showed higher average concentrations of Mn and Zn (31.08 and 11.02 mg/kg, respectively) especially for samples N32, N72, N101 (
Figure 1). These samples are near resorts, a boat station, workshops for general maintenance and body shops for marine transportation such as boats and jet skis. Moreover, many anthropogenic activities occur such as wastewater discharge and boat painting/repair around these stations which may cause the observed elevations in sediment metals concentrations [
12,
13,
42,
43]. Furthermore, most of the resorts and beach houses in the North location are still not connected to the municipal drainage network and discharge wastewater directly to the sea. Some authors [
19,
44,
45] report that discharge of wastewater may lead to the rise in concentrations of Mn in surface sediments. Zinc in the Northern stations may be due to pesticide usage, antifouling paints, detergents, and dispersant improvers for lubricating oil and antioxidants [
46,
47]. Most of the resorts are using pesticides to control pests and weeds which has been shown elsewhere to yield elevated Zn levels. In addition, batteries and the wear of automobile tires could be substantial anthropogenic sources of zinc near marine environments [
8,
47]. Indeed, most of the marinas use old tire products as anti-scratch material between the body of ships and the base of the marina.
Most of the metals were higher in the Middle location of the Jeddah coast (
Figure 1) than the other locations especially Pb, Mn, Zn and Cu (80.44, 76.94, 41.04, and 21.92 mg/kg, respectively) (
Table 2). According to Abu-Zied and Orif [
45], the marine environment in the vicinity of the northern stations of the Middle location (Stations M1 to M10) is used as a dumping zone for industrial and sewage wastewaters via sewage outflow pipes. This has led to the increase of organic matter which could act as a significant carrier phase for heavy metals and has been repeatedly shown to play a key role in the bioavailability, reactivity, and mobility of metals in sediments [
48]. In the present study, the prominent high concentration of Zn in the Middle location (Stations M6–M14) could be due to the outflow of organic matter from sewage sludge, industrial discharge, and antifouling paints [
19,
34].
The Middle location is located near the biggest desalination plant on the Jeddah coast. This plant is a major consumer of gasoline for onsite power production. The use of gasoline may be a possible source for the Pb in local sediments especially at stations M14, M15, M16, and M18 [
43] (
Figure 1). Elevated Mn concentrations recorded at stations M17–M20 occur near the main seaport of Jeddah; here, there are body shops for small fishing ships, where the residual ship wastes and antifouling paints are dominant. According to Abu-Zied et al. [
19], domestic sewage from the municipal drainage network of Jeddah City could be one of the reasons for elevated Mn concentrations. The high Cu concentrations in the sediments of the Middle stations may again be attributed to the availability of high amounts of organic matter (OM) in the sediments given its well-established high affinity for OM [
46,
47]. Additionally, a public park that was renovated and expanded between station M6 and station M15, may add a new source of OM to the location.
Pb was the most elevated heavy metal in the southern portion of the Middle location and particularly in the northern portion of the South location, reaching up to 600 mg/kg (
Figure 2). Most of the industrial activities of Jeddah City are located in the south and that could be identified as a primary potential source for pollutants at the studied southern stations (
Figure 2). Industrial contamination, therefore, is expected in the South location near to the industrial city. This complex is located near the northern end of the South location. As an example, a major petrol refinery is located near the northern stations of the South location. As expected from this interpretation, the most affected stations of the South location of the Jeddah coast are located in the northern part of this location. Additionally, station 7 is one of the most Pb-impacted stations due possibly to its proximity to the boat station and small local fish market.
The contamination factor analysis showed that about 40% of the middle stations were classified as being contaminated and very highly contaminated with Pb. Multiple sources of pollution at the Middle location of the Jeddah coast such as the desalination plant, industrial and sewage wastewater outflow pipes, the seaport, the fish market, and excessive shipping activities explain the north to south trends of PLI and RI values (
Figure 3). There is strong correlation between Pb concentrations and RI in the Middle location (r = 1.00). However, the correlation between Pb concentrations and PLI was the weakest of all metals (r = 0.67) at the Middle location, which may indicate that Pb is not controlling the pollution at this location (
Table 6). It is notable that both the PLI and RI trend higher as sampling progresses southward in the Middle location and both indices trend higher sampling northward in the South location (
Figure 3).
Measuring metals concentrations in sediment as a function to how distal the sediment is from shore can potentially provide useful information on likelihood of sourcing. The concentrations of Cr, Mn, Ni, Cu, and Zn were high for all the nearshore stations, however, Pb concentrations were higher for the offshore, deepest stations in the northern stations as well as in the central stations of transects of the southern part of Jeddah coast (
Table 7). The concentrations of Pb are known to increase offshore in the region [
44]. The current study generally shows a similar trend. Cr, Mn, Ni, Cu, and Zn trends all suggest sourcing from near-shore activities. The aberration of Pb increases with distance from shore in these regions, as expected from literature, and is also likely due to near shore sourcing. About 36% of the South location stations were classified as very highly contaminated with Pb via contamination factor calculations. Further, Pb was the most enriched metal in these stations, reaching up to 88%, which may indicate that the source of Pb is anthropogenic [
8].
Figure 3 shows that the PLI values for the stations S1–S4 were low; however, RI values were higher, which may indicate the occurrence of highest toxicity at these stations. The strong correlation of Pb with RI (r = 1.00) shows that the Pb is governing the pollution in the southern stations of Jeddah coast (
Table 6).
To further evaluate sourcing, we turn to the statistical analysis of the coexpression of metals. The bivariate correlations discussed above and shown in
Table 2 are supported by the multivariate statistical analysis. The MDS of the Northern location (
Figure 4a) shows that the distribution of metals is clustered into two groups, with S-Stress 0.012 and Tucker’s Coefficient of Congruence 0.99. The first group consists of Ni, Cu, Pb, and Cr. This group has a close similarity, indicating a similar source such as a siliciclastic influx (e.g., windblown dust) that accumulated along the coastline. The second includes Zn and Mn, which could be accumulated on the sediments from another source such as wastewater discharge or boat painting/repair waste that is discharged in this zone. For the Middle location, the MDS has also clustered the concentration of metals into two groups similar to the northern part with differentiation of the second group that includes Pb (
Figure 4b). This means that the lead concentration has been increased in this zone to reach a moderate contamination level, indicating to additional source for this metal (e.g., the desalination plant). However, the MDS of the Southern location shows that there is a high similarity among most of the metals in one group except Pb (
Figure 4c), suggesting a high accumulation of lead to high levels of contamination. This is could be due to industrial activities along the southern coast of Jeddah. These results are also confirmed by the PCA of the overall study area, which shows that Pb is clustered far away from other metals (
Figure 4d).
5. Conclusions
A total of 80 stations were chosen for collection of surficial marine sediments from North, Middle, and South locations of the nearshore Jeddah coast. The concentrations of six heavy metals (Cr, Mn, Ni, Cu, Zn, and Pb) in the collected sediments were determined. The average concentrations of these metals in surface sediments showed that Pb > Mn > Zn > Cr > Cu > Ni. The Enrichment Factors clearly show Pb was the most abundant heavy metal relative to background values in the study area. The Igeo values of the Pb in the Middle stations showed that 10% of the stations were categorized as moderately polluted, whereas 20% of the south locations were classified as moderately polluted. This is further supported by the contamination factor values as seen in
Table 4. Furthermore, Pb was the most enriched metal in the study area, which is consistent with an anthropogenic source of Pb. The RI values in the South stations, especially the northern stations within the South location, indicate high Pb pollution in the area.
The study results suggest the potential sources of pollutants in the Middle stations of the Jeddah coast could be anthropogenically sourced from released from the desalination plant, the outflow tubes of industrial and municipal wastewaters and waste of ship maintenance. The South location of the Jeddah coast, however, is interpreted to be polluted by the industrial activity wastes from the industrial city. This study recommends that appropriate management strategies should be applied for the North location of the Jeddah coast to control potential pollution sources and prevent permanent hazards to marine ecology currently documented elsewhere. This study indicates that the Middle location is impacted by various sources due to the highest activity in the region. Stringent management practices are suggested to limited further metals contamination to the area. The Southern location of the Jeddah coast is the most toxic location; it, therefore, needs more effort and strong regulations to treat and recover the marine environment. The area needs more research towards exact sourcing of contaminants, specifically lead. A further study of metals speciation in marine sediments of Jeddah coast should be carried out to yield more information about the bioavailability and mechanism of deposition of these metals and their sourcing. Moreover, tracer studies, such as isotopic studies, paired with Pb speciation could be used to investigate the sources of the lead in sediments, water, and even in airborne aerosols near to the shoreline.
Heavy metals in near-shore marine sediments and their associated presence in marine waters are a threat to marine ecosystems. A comprehensive evaluation of heavy metals contamination in such sediments, therefore, is prudent for sensitive regions in close proximity to development. The results of this study demonstrate the use of different environmental indicators to identify current environmental health status and risk associated with anthropogenic activities. Further, bi- and mutlivariant statistical techniques suggest sourcing of coexpressed pollutants. The findings will provide a valuable set of benchmarks for future research. Additionally, they will be a highly valuable guide to environmental decision makers in KSA and counterparts in other regional countries. Finally, the techniques employed and the correlations demonstrated can further aid similar work in analogous ecosystems worldwide.